We have characterized six identical clones from a rat uterus cDNA library assembled in the expression vector, lambda gtll and screened with affinity purified antibodies to myosin light chain kinase (MLCK). That these clones contain a 630 base pair cDNA for MLCK is suggested by: (1) the clones were isolated using a highly specific antibody probe for MLCK; (2) the clones express a peptide that cross-reacts with antibodies to MLCK in an epitope selection analysis; (4) hybrid selection studies indicate synthesis of a 100,000 MW protein; (5) immunoprecipitation of in vitro translation products of uterine messenger RNA (mRNA) identify a 100,000 MW protien; and (6) the 630 bp probe hybridizes to a 2.5 kb band on a northern blot of rat uterine total RNA. We have recently isolated a 2.4 kb cDNA from a lambda gtll rat uterus library screened with the affinity purified antibody to MLCK. Preliminary studies indicate: (1) this cDNA does not hybridize to the 630 bp clone; (2) this clone appears positive for MLCK on epitope selection and (3) the cDNA may cross-hybridize to an oligonucleotide probe constructed on the basis of the amino acid sequence near the site phosphorylated by cAMP-dependent protein kinase in turkey gizzard MLCK.